As an electric power steering apparatus for assisting a steering power by use of a motor, for example, an electric power steering apparatus for braking the motor in the case where it is desirable to reduce the steering torque as in the high-speed running mode has previously been known.
Such a type of conventional apparatus, for example, has been known as an electric power steering apparatus disclosed in, e.g., Japanese Patent Public Disclosure (KOKAI) No. 59-130781 in 1984.
In such a conventional apparatus, when a load is connected to the coil of the motor, a current corresponding to a magnitude of the load flows through the coil. A power opposite to the force which is applied from the outside, i.e., the braking power (power generation braking) is produced. By use of this braking power, the steering torque is controlled so as to coincide with a reference torque. Practically speaking, by pulse-width controlling of the time when a resistive element as a load is connected to the motor, the braking power is adjusted in accordance with the steering condition.
In a conventional motor braking circuit, a pulse-width modulation (PWM) signal indicative of the steering torque is adjusted and a braking current is allowed to flow through a resistive element, as mentioned above. In such a motor braking circuit, if the steering wheel was suddenly stopped during the steering operation in the running mode, the value of the electric power energy, which is generated by the rotation of the motor due to the reactive rotation of the motor or the reverse thrust of the rack of the steering mechanism, is relatively small. Therefore, this electric power energy, i.e. the braking current cannot be sufficiently absorbed by the resistive element, so that yawing of the vehicle body occurs, causing the steering operability of the steering wheel to remarkably deteriorate.